Two die halves are used in die casting. One is a movable die that moves from one position to another, and the other is a fixed location die. Where the two dies meet is called the parting line. The fixed die contains the shot hole, which allows molten metal to flow into the dies. The movable die contains ejector pins and usually the gates, which are the pathways from the shot hole to the mold cavity. The mold cavity may consist of removable cavity inserts, which are separate pieces that can be replaced relatively easily and bolt into the die halves.
The dies are designed so that the finished casting will slide off the fixed half of the die and stay in the movable half as the dies are opened. This assures that the casting will be ejected every cycle because the movable half contains the ejector pins to push the casting out of that die half. The ejector pins are driven by an ejector plate, which accurately drives all of the pins at the same time and with the same force, so that the casting is not damaged. The ejector plate also retracts the pins after ejecting the casting to prepare for the next shot. There must be enough ejector pins to keep the overall force on each pin low, because the casting is still hot and can be damaged by excessive force. The pins still leave a mark, so they must be located in places where these marks will not hamper the casting's purpose.
A die release agent may be applied to the inside surfaces of the die between each shot. The die release agent prevents buildup of material on the die. This spray also causes some cooling of the die.
Other die components include cores and slides. Cores are components that usually produce holes or openings, but they can be used to create other details as well. There are two types of cores: fixed and movable. Fixed cores are ones that are oriented parallel to the pull direction of the dies (i.e. the direction the dies open); therefore they are fixed, or permanently attached to the die. Movable cores are ones that are oriented in any other way than parallel to the pull direction. These cores must be removed from the die cavity after the shot solidifies, but before the dies open, using a separate mechanism. Slides are similar to movable cores, except they are used to form undercut surfaces. Other features in the dies include water-cooling passages and vents along the parting lines. These vents are usually wide and thin (approximately 0.13 mm or 0.005 in) so that when the molten metal starts filling them the metal quickly solidifies and minimizes scrap. No risers are used because the high pressure ensures a continuous feed of metal from the shot.
Important material properties for the dies include thermal shock resistance and softening at elevated temperature, hardenability, machinability, heat checking resistance, weldability, availability (especially for larger dies), and cost. The longevity of a die is directly dependent on the temperature of the molten metal and the cycle time. The dies used in die casting are usually made out of hardened tool steels, because cast iron cannot withstand the high pressures involved, therefore the dies are very expensive, resulting in high start-up costs. Metals that are cast at higher temperatures require dies made from higher alloy steels. Due to the cost and difficulty of manufacturing dies, there is identified a need to manage and maximize the life cycle of the dies.